Duct heat exchanger

ABSTRACT

A duct comprising: an inlet; an outlet; a shell having a tubular form extending between the inlet and the outlet; a main flow path (H) within the shell for conveying a main flow between the inlet and the outlet; and a heat exchange structure, wherein the heat exchange structure comprises: an intake port provided in the shell; an output port provided in the shell; and a secondary flow path (C) within the shell for conveying a secondary flow between the intake port and the output port, wherein the secondary flow path is spirally intertwined with the main flow path for a section of the duct to provide a heat exchanger within the duct.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.19461504.3 filed Jan. 15, 2019, the entire contents of which isincorporated herein by reference.

FIELD

The disclosure relates to a duct that includes a heat exchanger and amethod of manufacturing the same.

BACKGROUND

Heat exchangers are required within aircraft structures to regulatetemperatures of working fluids as well as to scavenge heat from onesystem for use in another. Every heat exchanger consumes space withinsuch an aircraft structure. They also usually need complicated mountingfeatures and they will generally add weight to the aircraft structure.There is a desire to solve one or more of these problems.

SUMMARY

Viewed from one aspect, the present disclosure can be seen to provide aduct comprising an inlet, an outlet, a shell having a tubular formextending between the inlet and the outlet, a first flow path within theshell for conveying a first flow between the inlet and the outlet, and aheat exchange structure. The heat exchange structure comprises an intakeport provided in the shell, an output port provided in the shell and asecond flow path within the shell for conveying a second flow betweenthe intake port and the output port. In the duct, the second flow pathis spirally intertwined with the first flow path for a section of theduct to provide a heat exchanger within the duct.

In addition to the features described above, the first flow path and thesecond flow path may comprise intertwined spiral channels that areformed by a spiral wall arranged within the shell.

In addition to one or more of the features described above, the spiralwall may extend radially from an outer surface of a spine to an innersurface of the shell.

In addition to one or more of the features described above, the spiralwall may have a first surface in contact with the first flow path and asecond surface in contact with the second flow path. This relationshipof the first surface being in contact with the first flow path and thesecond surface being in contact with the second flow path, may bemaintained along the length of the heat exchanger.

In addition to one or more of the features described above, the spiralwall may provide a double helix structure within the shell for thespiral channels with a pitch angle of less than 20°. Optionally thepitch angle may be less than 10°. The pitch angle may be constant alongthe length of the heat exchanger.

In addition to one or more of the features described above, the shellmay comprise one or more bends between the inlet and the outlet.Optionally the shell may comprise a bend in a first direction and a bendin a second direction which is different to the first direction toprovide a duct with an S-shaped shell.

In addition to one or more of the features described above, the spiralchannels may comprise a plurality of helical turns and a thicknessdirection of the spiral wall may be varied to maintain a uniformcross-sectional flow area in each of the spiral channels through all thehelical turns around a bend. Optionally the spiral wall may compriseinternal blind cavities, for example, closed cells extending within aradial extent of the spiral wall. Optionally the spiral wall maycomprise a thickness which increases from an inside of a bend to anoutside of a bend. The thickness of the spiral wall at the outside ofthe bend may be greater than the thickness of the spiral wall for astraighter section. In addition or alternatively, for a straight sectionof the shell, a constant thickness of the spiral wall may be maintained.

In addition to one or more of the features described above a radialextent of a channel may be less at an outside of a bend than at aninside of the bend to maintain a uniform cross-sectional flow area ineach of the spiral channels through all the helical turns around a bend.

In addition to one or more of the features described above, the duct maycomprise a first section downstream of the inlet in which the shelldefines a first portion of the first flow path where the second flowpath is absent, a second section downstream of the first section andpositioned between the output port and the intake port comprising theheat exchanger where the first flow path and the second flow path arepresent and spirally intertwined, and a third section downstream of theheat exchanger and leading to the outlet where the shell defines asecond portion of the first flow path where the second flow path isabsent.

In addition to one or more of the features described above, the intakeport and the output port may be provided with necks which extend fromthe shell for connection to a supply and return of heat exchangermedium.

In addition to one or more of the features described above, the heatexchanger may comprise a spine formed by inner portions of the firstflow path and the second flow path that extends in a longitudinaldirection of the duct. Optionally the spine may be hollow and comprisesa valve to provide a bypass flow for the first flow path. The valve maybe activated automatically by excessive pressure or via a controlsystem.

In addition to one or more of the features described above, one or moreof a spiral wall, a spine or the shell, which together define a flowpath area of a flow path within the heat exchanger, comprise one or moreblind cavities internally within the spiral wall, spine and/or shell.Optionally wherein, the one or more blind cavities are provided to takeaccount of different flow rates within the first and second flow paths.

In addition to one or more of the features described above, the duct,including the heat exchanger provided by the spirally intertwined firstflow path and second flow path, may have been formed by an additivemanufacturing process. Optionally the additive manufacturing process maybe a laser bed fabrication process.

Viewed from another aspect, the present disclosure can be seen toprovide a method of making a duct comprising forming a shell having atubular form to define an inlet and an outlet of the duct, the ductproviding a first flow path within the shell for conveying a main flowbetween the inlet and the outlet, forming a heat exchange structurewithin the duct, the heat exchange structure comprising an intake portformed in the shell, an output port formed in the shell and a secondflow path formed within the shell for conveying a second flow betweenthe intake port and the output port. In the method, forming the heatexchange structure comprises forming the second flow path so that it isspirally intertwined with the first flow path for a section of the ductto provide a heat exchanger within the duct.

In addition to the features described above, the duct may be formed byan additive manufacturing process. The additive manufacturing processmay be a laser bed fabrication process or similar process comprising alaser which is used to fuse powder particles together to build the duct.The forming of the heat exchange structure may take place concurrentlywith the forming of the shell in such a fabrication process.

In addition to one or more of the features described above, a spiralwall may be formed within the shell of the duct to provide an internalstructure having a double helix form of spiral channels. Optionally athickness of the spiral wall may be varied such that the thickness ofthe spiral wall is thinner towards an inside of a bend in the shell, andis wider towards an outside of the bend in the shell.

The present disclosure may also extend to an aircraft comprising one ormore ducts comprising a heat exchanger as described above. The one ormore ducts comprising a heat exchanger may be part of an OBIGGS unitwithin the aircraft.

BRIEF DESCRIPTION OF FIGURES

Certain embodiments will be described below in greater detail by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a conventional plate fin heat exchanger;

FIG. 2 is a perspective view of a duct incorporating a heat exchanger inaccordance with an exemplary embodiment of the present disclosure;

FIG. 3 is a perspective cut-away view showing a portion of a ductincorporating a heat exchanger in accordance with an exemplaryembodiment;

FIG. 4 is a perspective cut-away view showing a portion of a ductincorporating a heat exchanger in accordance with an exemplaryembodiment;

FIG. 5 shows a perspective end view of a portion of a duct incorporatinga heat exchanger in accordance with an exemplary embodiment;

FIG. 6 shows a cross-section of a bend portion (exaggerated) of a ductwith a heat exchanger; and

FIG. 7 shows a cross-section of a similar bend portion with thethickness of the channel walls modified in accordance with an exemplaryembodiment.

DETAILED DESCRIPTION

FIG. 1 shows a conventional plate fin heat exchanger 10 for use withinan aircraft system. In the figure, the plate fin heat exchanger 10 has afirst flow path for a hot fluid H and a second flow path for a coldfluid C. The hot and cold fluid flow paths H, C enter and exit the heatexchanger 10 via the inlets 12 a, 12 b and outlets 14 a, 14 b, therespective inlets and outlets being arranged opposite each other on ahousing 16. The hot fluid H is conveyed to and from the heat exchanger10 by a pair of ducts 13 a, 13 b, and the cold fluid C is conveyed toand from the heat exchanger 10 by a pair of ducts 15 a, 15 b.

Within the housing 16 of the heat exchanger 10, the hot and cold fluidflow paths H, C intersect through a series of heat exchanger plates (notshown) to transfer heat. The exchange of heat might be, for example, toregulate the temperature of the hot fluid by extracting heat with thecold fluid. In the illustrated heat exchanger 10, the heat exchangerplates are located within the box-shaped, central part 16 a of thehousing 16, and the fluid flows H, C are divided across a plurality ofintersecting channels by headers provided in the curved portions 16 b ofthe housing 16, arranged every 90° around the housing 16.

This type of heat exchanger 10, while it is thermodynamically efficient,it consumes a significant amount of space within an aircraft structure.It also adds weight to the aircraft structure and usually requirescomplex mounting arrangements. For example, a mounting arrangement mayneed to be provided within an already congested space for supporting theheat exchanger 10 via bracket 18.

In an aircraft, it is beneficial to keep component size and weight to aminimum. There may also be fewer options for mounting the heat exchanger10 in an aircraft.

It may also be desirable to transfer heat from one system to another,for example, where efficiencies can be improved through regulating ormodifying the temperature in a flow of working fluid, or where heat (orcooling) can be scavenged from one system to the advantage of another.However, often there is not the space available for mounting aconventional plate fin heat exchanger within the existing aircraftstructure.

According to the present disclosure, the conventional heat exchanger isreplaced with a modified duct 20, as shown in FIG. 2 , whichincorporates an integral heat exchanger. In this way, less space isneeded than for the conventional heat exchanger 10.

Moreover, the new duct 20 allows for an existing duct in an aircraftstructure to be improved by replacing it for a duct 20 as describedherein; in order to offer heat exchange functionality without requiringadditional space and/or additional mounting points.

The duct 20 comprises a tubular shell 30 that can be shaped to wraparound and fit past other components in a region which is restricted onspace. The duct 20 with the internal heat exchanger needs only to takeup the room occupied by an existing duct within an aircraft structure.

The duct 20 may comprise a non-regular shape, for example, with one, twoor more bends 20 a, 20 b along its length. In the illustrated example ofFIG. 2 , the duct 20 follows an S-shape comprising two main bends 20 a,20 b in different directions. However, the current disclosure is notlimited to such shapes and other configurations (simpler as well as morecomplex) are contemplated.

In FIG. 2 , the tubular shell 30 of the duct 20 is shown of beingsubstantially constant diameter along its length. This would be atypical solution so that the duct 20 can fit within the space available.However, it would also be possible for the duct 20 to be other shapes,particularly at the ends, where it may not be truly circular incross-section for all or part of its length. For example, depending onthe spacing of neighbouring components, it may be possible to form theshell 30 to a more organic shape than shown to make use of availablespace. Moreover, for reasons that will become apparent below, the shell30 may have a slightly distorted shape to help maintain a more uniformfluid flow around a bend 20 a, 20 b. It may comprise a slight bulge orother distortion too, while maintaining a smooth spiral flow within theheat exchanger.

Regardless of the specific configuration, the shell 30, should generallystill have an overall external form which would be consistent with thatof a duct, albeit modified to provide the heat exchange functionality.

Thus the duct 20 of the present disclosure, can be seen from one aspectas a duct heat exchanger. It is a duct with an internal heat exchanger.

The duct 20 can be shaped to follow an existing path within the aircraftstructure, for example, where it is being used to replace a previousduct without heat exchange functionality. The duct 20 can also rely onexisting fixing points, avoiding the need for complicated fixingstructures. Incorporating the heat exchanger within the existingenvelope of a duct can also help to reduce the weight of the heatexchanger arrangement compared to fitting a conventional heat exchangerof larger dimensions.

Thus the duct 20 has an outer shell 30 which encloses a heat exchanger35, which will be described in more detail below. The shell 30 istubular in nature and extends from an inlet 22 to an outlet 24, forexample, as shown in FIG. 2 . The shell 30 contains and directs a firstflow path, for example, a main flow path, as it travels along the duct20 between the inlet 22 and outlet 24. In the embodiment of FIG. 2 thisis indicated as a hot fluid flow H.

The duct 20 also comprises an intake port 26 and an output port 28 inthe shell 30. The intake port 26 and the output port 28 are provided byorifices in the circumferential surface defined by the shell 30. Asecond flow path, for example, a secondary flow path for a cold fluidflow C, is then defined between the intake port 26 and the output port28 to provide a heat exchanger 35 within the shell 30.

In this exemplary embodiment, the main flow path is the first flow pathand is for a hot fluid H and the secondary flow path is the second flowpath and carries the cold fluid C to extract heat and regulate thetemperature of the hot fluid H; however it will be appreciated that thefirst/main flow path could carry the cold fluid C and thesecond/secondary flow path could be provided with a hot fluid H to inputheat into the cold fluid C.

The embodiment in FIG. 2 also shows the first/main and second/secondaryflow paths H, C flowing in opposite directions within the heat exchanger35 to increase the efficiency of the heat exchanger 35. It will beappreciated that it is possible for both flow paths to be in the samelongitudinal direction. In the description below, in relation to theflow paths and flows contained therein, the terms “first” and “main” canbe used interchangeably, and similarly “second” and “secondary” can beused interchangeably.

Also the intake port 26 and the output port 28 may be provided atdifferent positions to that shown on the shell 30, for example, closerto or further from an end 21 a, 21 b of the duct 20, as desired.

The intake port 26 and the output port 28 may include extended portionsproviding necks 26 a, 28 a as shown to facilitate connection with otherducts conveying the secondary flow C. The necks 26 a, 28 a may extendfrom the outer surface of the shell by 10 mm or more. While the necks 26a and 28 a are shown comprising rectangular shaped apertures at theirdistal ends 26 b, 28 b, the cross-section of these necks 26 a, 28 acould vary, for example, to provide round or ovalised apertures at thedistal ends 26 b, 28 b, as desired.

The duct 20 with the heat exchange functionality may be used with anycombination of fluids, such as liquid-liquid, liquid-gas or gas-gas heatexchange. In the context of aerospace, the fluids are most likely to begaseous, i.e., that the duct 20 would comprise a main flow which is agas and a secondary flow to regulate or modify the working temperatureof the main flow which is also a gas. It could include two or more of:atmospheric air, cabin air, engine gas flow, exhaust flow, engine oil,generator oil, coolant, fuel and so on.

FIG. 3 shows an exemplary embodiment of a portion of the duct 20 withthe shell 30 partially removed to display the internal structure of theduct 20. The secondary flow path C is spirally intertwined with the mainflow path H, such that they provide a section of the duct 20 with a heatexchanger 35.

The heat exchanger 35 is formed by the secondary flow path extendingbetween the intake port 26 and the output port 28, providing the heatexchange structure. The heat exchanger 35 may extend along substantiallythe entire length of the duct 20, for example, greater than 60%, or even75% of the length of the duct 20. In one example, the section comprisingthe heat exchanger 35 represents over 85% of the length of the duct 20.

Thus in the embodiment of FIG. 2 , the duct 20 can be seen to comprisethree sections. A first section 36 downstream of the inlet 22 in whichthe shell 30 defines a first portion of the main flow path H where thesecondary flow path is absent. A second section of the duct 20downstream of the first section 36 and positioned between the outputport 28 and the intake port 26 comprises the heat exchanger 35 where themain flow path H and the secondary flow path C are present and spirallyintertwined. A third section 37 of the duct 20 downstream of the heatexchanger 35 and leading to the outlet 24 where the shell 30 defines asecond portion of the main flow path H where the secondary flow path isabsent.

Depending on the thermal requirements of the duct 20 and theconfiguration of the surrounding aircraft structure, the heat exchanger35 may extend along a smaller section of the duct 20, or conceivably, asecond or further heat exchanger 35 may be formed within the duct 20through the provision of additional intake and output ports 26, 28 (notshown).

As can be seen in FIG. 3 , the main flow path and the secondary flowpath are intertwined such that they form a double helix structure withtwo spiral channels 32, 34 within the duct 20. The main flow path passesalong one of the spiral channels 32 and the secondary flow path passesalong the other spiral channel 34. The pair of spiral channels areseparated by a spiral wall 33 forming the double helix structure of theheat exchanger 35 and dividing the flow paths H, C.

At each point along the heat exchanger 35, the spiral wall 33 may have afirst surface 33 a in contact with the main flow path H and a secondsurface 33 b in contact with the secondary flow path C. Embodiments arealso envisaged where an additional heat exchanger fluid is providedwithin the duct 20, for example, within a further intertwined spiralchannel within the heat exchanger 35.

The intake port 26 shown in FIG. 3 is for the secondary flow C and mayproject outwardly from the surface of the shell 30 to form the neck 26 ashown in FIG. 2 . The intake port 28 is rectangular in cross section,corresponding to the cross-section of the spiral channel 34, and offsetto one side of the duct axis A-A. The output port 28, not visible inFIG. 3 , could comprise a similar configuration.

As can be seen from this figure, the heat exchanger 35 is effectivelyprovided by the three-dimensional spiral form of the secondary flow pathC provided within the shell 30. The main flow path H in this sectionwith the heat exchanger 35 can be seen as the remaining flow areabetween the turns of the secondary flow path C.

The duct 20 shown in FIG. 3 is a straight duct. However, in practice,the duct 20 may usually comprise a more organic shape, for example, theduct 20 may comprise one or more bends 20 a, 20 b along its length suchthat it can fit around other components within the system, for example,an S shape as shown in FIG. 2 .

FIG. 4 shows an outlet end 21 b of an exemplary embodiment of the duct20 in more detail, and in particular the flow path of the cold fluidflow C within the heat exchanger 35. In this embodiment the main flow Hspirals in an anticlockwise direction through the spiral channel 32 ofthe heat exchanger 35 to exit via outlet 24. By contrast, the secondaryflow C enters the shell 30 through the intake port 26 from the neck 26 aand follows the spiral channel 34 in the opposite spiral direction tothe main flow H, in this case a clockwise spiral direction indicated bythe arrows in the figure. Heat transfer occurs between the two fluidsalong the heat exchanger 35 and the secondary flow exits the shell 30via output port 28.

The intake port 26 and output port 28 may be at opposite or non-alignedorientations.

The spiral directions can of course be reversed, and the secondary flowC could follow an anti-clockwise spiral direction while the main flow Hfollows a clockwise spiral direction.

FIG. 5 shows a perspective end view of a duct 20 illustrating the outputport 28 and the intersection of the secondary flow path C with the shell30 in more detail. Looking through the inlet 22 of the main flow path H,it is possible to see the start of the spiral channel 34 for thesecondary flow path C which is defined by the spiral wall 33 dividingthe flows. Between the turns of the spiral channel 34, the main flowpath H will follow an intertwined helical path within its own spiralchannel 32 defined in part by the spiral wall 33.

The rear portion of the duct 20 illustrated in the figure is shown withthe shell 30 omitted to show the intertwined spiral channels 32, 34continuing within the duct 20. The intake port 26 (not shown) of theduct 20 may be configured in a similar way.

The spiral wall 33 forming the spiral channels 32, 34 may comprise thesame helical pitch along the length of the heat exchanger 35. In asection of the duct 20 with a constant radius, this will maintain aconstant cross-sectional area of spiral passage for the respectivespiral channels 32, 34. Where it is desirable to vary a flow rate withinthe heat exchanger 35, the helical pitch may be increased or decreasedas appropriate. The helical pitch of one spiral channel 32 maycorrespond to the helical pitch of the other spiral channel 34 sincethey are intertwined.

The spiral wall 33 may provide a double helix structure within the shellfor the spiral channels by dividing the flows. The spiral wall 33 mayprovide a double helix structure with a pitch angle α of less than 20°.The pitch angle α may be less than 10°. The spiral channels 32, 34 mayhave the same axial depth along the heat exchanger 35, i.e., that thedistance between the surfaces of the spiral wall 33 for each channel 32,34 in the longitudinal direction is kept the same. The radial width ofthe spiral channels 32, 34 may correspond to the radial distance betweenthe shell 30 and a spine 40 extending centrally within the duct 20. Thechannels may have substantially rectangular flow areas, defined on twosides by the surfaces of the spiral wall 33 and on the other two by aportion of the spine 40 and a portion of the shell 30. The channels 32,34 could comprise rectangular flow areas with rounded corners. Thecross-sectional flow area of the main flow path may be the same as thecross-sectional flow area of the secondary flow path, though it wouldalso be possible for the flow areas to be different.

As shown in FIG. 4 , the spiral channels 32, 34 may be wrapped aroundthe spine 40 to form the double helix structure. The spine 40 canprovide a supporting member or back bone for both the shell 30 of theduct 20 and the spirally intertwined channels 32, 34 formed around it.In this way, the internal structure can be self-supporting and transferload from the heat exchanger 35 to the ends 21 a, 21 b of the duct 20where the conventional fixing points are usually located.

The width of the spine 40 is defined by the inner diameter of the spiralchannels 32, 34. In the exemplary embodiment, these are shown with thesame inner diameter, but it would be possible to have different innerdiameters to accommodate different flow requirements.

In a similar way the inner surface of the shell 30 for one channel 32,34 may be a different internal radius than for the other of the channels32, 34 to accommodate different flow requirements.

The different internal diameters and radii could be provided by formingthe spine 40 and/or shell 30 with different wall thicknesses, forexample, when 3D printing the duct 20. However the different wallthicknesses could also be provided through the provision of internalblind cavities in order to reduce the overall weight of the part.

In one exemplary embodiment, the spine 40 may be hollow in order to actas a by-pass channel in the event that a blockage occurs in the mainflow path or when a heat exchange function is not required for a givenalong fluid. FIGS. 6 and 7 show the spine 40 as a hollow tube.

The duct 20 can be formed by additive manufacturing methods, forexample, by using laser bed fabrication. This process enables complexshapes such as the double helix internal structure of the heat exchanger35 to be formed simply. Due to the relatively low pitch angle of thedouble helix channels (e.g., less than 20°), the duct 20 can be printedeasily without requiring additional internal supports. There istherefore some synergy with the configuration of the internal structureand additive manufacturing fabrication. In particular the radial extentof the spiral wall 33, the helical path of the spiral channels andtubular nature of the shell 30 allows the duct 20 to be fabricatedeasily, forming the heat exchanger 35 in combination with the shell 33when building the duct in a longitudinal direction (for example, withthe axis A-A extending substantially vertically from a bed of powder ina laser bed fabrication apparatus. However, conceivably the duct couldalso be made using conventional manufacturing methods, for example,through being cast in two halves and joined together or through aninvestment casting technique.

The use of additive manufacturing in particular allows the duct to beshaped to fit a specific space without requiring new moulds. Forexample, it may be formed with one or more bends 20 a, 20 b in order tofit within an existing aircraft system. Thus the duct 20 can be tailoredin its configuration to fit within an existing space within the aircraftsystem, following a path through existing components. It can also beformed with suitably shaped profiles at the inlet/outlet 22, 24 and/orintake port/output port 26/28, for example, a circumferentiallyextending rib (not shown).

FIG. 6 shows, in cross-section, an exaggerated illustration of a bend 20a in the duct 20. On the left-hand side of FIG. 6 , the spiral channels32, 34 have a uniform, substantially rectangular cross-section. As theyfollow around the bend 20 a the cross-sectional shape changes to followthe curvature of the bend 20 a. The spiral wall 33 in this arrangementthen defines a tapering space with a narrow end d1 at the inside 50 ofthe bend and a wider end d2 at the outside 52 of the bend 20 a such thatd1 is less than d2 in FIG. 6 .

In order to avoid pressure differences within the heat exchanger 35 theinternal structure of the heat exchanger 35 may be adapted to maintain auniform cross-sectional flow area in each of the spiral channels 32, 34.Such adaptions can be easily incorporated into the design of a duct 20which is made by additive manufacturing.

In one exemplary embodiment, the thickness t of the spiral wall 33 maybe varied in order to maintain a constant cross-sectional flow area inthe spiral channels 32, 34.

FIG. 7 shows an exemplary embodiment of a similar bend 20 a in the duct20 but where the thickness t of the spiral wall 33 varies from a minimumt1 at the inside 50 of the bend 20 a to a maximum t2 at the outside ofthe bend 52, i.e., t1<t2. The variation in thickness t may be a linearrelationship, such that the spiral wall 33 appears as a tapered shape,for example, a triangular or pointed shape, when viewed in cross-sectionas shown in FIG. 7 . Such an adaption can be easily accomplished withadditive manufacturing to provide a constant cross sectional flow areain each of the spiral channels 32, 34 along the length of the heatexchanger 35.

As an alternative, the spiral walls 33 may be hollow or comprise blindcavities. This can easily be incorporated into the design of the duct 20and will reduce the overall weight of the duct 20. The presence ofhollow spiral walls 33 or walls comprising internal blind cavities alsoallows for the cross sectional flow area of one or both of the main flowpath H or secondary flow path C to be varied, for example, to takeaccount of different flow rates within the flow paths H, C.

Alternatively or in addition, the radial extent r of the spiral channels32, 34 may be varied around the bend such that the cross sectional flowarea of each of the spiral channels 32, 34 remains uniform as thehelical turns extend around the bend 20 a. This can be achieved throughaltering the configuration of the shell 30 by drawing it out from thecentre on the inside 50 of the bend 20 a or by drawing it in towards thecentre on the outside 52 of the bend 20 a, to adjust the cross-sectionalflow area to make it more uniform. Thus the shell 30 may be viewed ashaving a narrower radius at the outside 52 of the bend 20 a compared tothe inside 50 of the bend 20 a, when taking the spine 40 as followingthe axis A-A of the duct. The shell 30 may comprise a modifiedcross-section in a bend which is not circular; it may appear flattenedon the outer surface.

Viewed another way the position of the spine 40 may be displaced fromthe axis A-A closer to the outside 52 of the bend 20 a than the inside50, in order to keep the cross-sectional area of the spiral channels 32,34 as uniform as possible while maintaining a circular cross-section.

In both scenarios, the relative position of the spine 40 with respect tothe shell is effectively adjusted, so that a radial extent r of eachchannel r1 at a portion 40 a of the spine 40 on the inside 50 of thebend 20 a may be greater than a radial extent r2 on the outside 52 ofthe bend 20 a between a portion 40 b of the spine 40 and the shell onthe outside 52 of the bend 20 a.

The duct 20 may be made by additive manufacturing using a range ofmaterials. For example, the duct 20 may be formed from an aluminiumalloy for lower temperature applications, offering good thermalconductivity properties, or it may be formed from a superalloy materialsuch as (but in no way limited to) Inconel 718 for use under highertemperature conditions.

Providing the duct 20 with heat exchanger functionality offers benefitsin terms of the overall size of the heat exchanger arrangement. Forexample, it is able to be designed to fit within the space correspondingto a conventional duct. This may offer benefits and new opportunitiesfor transferring heat between systems that were not previously possible.

The duct 20 with the internal heat exchanger 35 may be used by itself orin conjunction with other such ducts 20 in an aircraft to replace one ormore conventional heat exchangers, for example, plate-fin heatexchangers. The one or more ducts 20 comprising the internal heatexchanger 35 may be part of an OBIGGS unit within the aircraft, forexample, or a similar unit where the heat exchange functionality is ofbenefit.

The invention claimed is:
 1. A duct comprising: an inlet; an outlet; ashell having a tubular form extending between the inlet and the outlet;a first flow path within the shell for conveying a first flow betweenthe inlet and the outlet; and a heat exchange structure, wherein theheat exchange structure comprises: an intake port provided in the shell;an output port provided in the shell; and a second flow path within theshell for conveying a second flow between the intake port and the outputport, wherein the second flow path is spirally intertwined with thefirst flow path for a section of the duct to provide a heat exchangerusing the heat exchange structure of the duct; wherein the shellcomprises one or more bends between the inlet and the outlet, and thefirst flow path and the second flow path comprise intertwined spiralchannels that are formed by a spiral wall arranged within the shell;wherein the one or more bends comprises a bend in a first direction anda bend in a second direction which is different to the first directionto provide a duct with an S-shaped shell; and wherein a radial extent ofa channel is less at an outside of a bend of the one or more bends thanat an inside of the bend to maintain a uniform cross-sectional flow areain each of the spiral channels through all the helical turns around thebend.
 2. A duct as claimed in claim 1, wherein the spiral wall extendsradially from an outer surface of a spine to an inner surface of theshell.
 3. A duct as claimed in claim 1, wherein the spiral wall has afirst surface in contact with the first flow path and a second surfacein contact with the second flow path.
 4. A duct as claimed in claim 1,wherein the spiral wall provides a double helix structure within theshell for the spiral channels with a pitch angle of less than 20°.
 5. Aduct as claimed in claim 4, wherein the pitch angle of less than 10°. 6.A duct as claimed in claim 1, wherein the duct comprises: a firstsection downstream of the inlet in which the shell defines a firstportion of the first flow path where the second flow path is absent; asecond section downstream of the first section and positioned betweenthe output port and the intake port comprising the heat exchanger wherethe first flow path and the second flow path are present and spirallyintertwined; and a third section downstream of the heat exchanger andleading to the outlet where the shell defines a second portion of thefirst flow path where the second flow path is absent.
 7. A duct asclaimed in claim 1, wherein the intake port and the output port areprovided with necks which extend from the shell for connection to asupply and return of heat exchanger medium.
 8. A duct as claimed inclaim 1, wherein the heat exchanger comprises a spine formed by innerportions of the first flow path and the second flow path that extends ina longitudinal direction of the duct.
 9. A duct as claimed in claim 1,wherein the duct, including the heat exchanger provided by the spirallyintertwined first flow path and second flow path, has been formed by anadditive manufacturing process.
 10. A duct as claimed in claim 8,wherein the spine is hollow and comprises a valve to provide a bypassflow for the first flow path.
 11. A duct as claimed in claim 9, whereinthe additive manufacturing process is a laser bed fabrication process.12. A duct comprising: an inlet; an outlet; a shell having a tubularform extending between the inlet and the outlet; a first flow pathwithin the shell for conveying a first flow between the inlet and theoutlet; and a heat exchange structure, wherein the heat exchangestructure comprises: an intake port provided in the shell; an outputport provided in the shell; and a second flow path within the shell forconveying a second flow between the intake port and the output port,wherein the second flow path is spirally intertwined with the first flowpath for a section of the duct to provide a heat exchanger using theheat exchange structure of the duct; wherein the shell comprises one ormore bends between the inlet and the outlet, and the first flow path andthe second flow path comprise intertwined spiral channels that areformed by a spiral wall arranged within the shell; and wherein thespiral channels comprise a plurality of helical turns and a thickness ofthe spiral wall is varied to maintain a uniform cross-sectional flowarea in each of the spiral channels through all the helical turns arounda bend of the one or more bends.
 13. A duct as claimed in claim 12,wherein the spiral wall comprises a thickness which increases from aninside of a bend to an outside of a bend, or wherein the spiral wallscomprise blind cavities.
 14. A duct as claimed in claim 12, wherein aradial extent of a channel is less at an outside of a bend than at aninside of the bend to maintain a uniform cross-sectional flow area ineach of the spiral channels through all the helical turns around a bend.15. A duct as claimed in claim 12, wherein the spiral wall extendsradially from an outer surface of a spine to an inner surface of theshell.
 16. A duct as claimed in claim 12, wherein the spiral wall has afirst surface in contact with the first flow path and a second surfacein contact with the second flow path.
 17. A duct as claimed in claim 12,wherein the spiral wall provides a double helix structure within theshell for the spiral channels with a pitch angle of less than 20°.
 18. Aduct as claimed in claim 17, wherein the pitch angle of less than 10°,wherein the duct comprises: a first section downstream of the inlet inwhich the shell defines a first portion of the first flow path where thesecond flow path is absent; a second section downstream of the firstsection and positioned between the output port and the intake portcomprising the heat exchanger where the first flow path and the secondflow path are present and spirally intertwined; and a third sectiondownstream of the heat exchanger and leading to the outlet where theshell defines a second portion of the first flow path where the secondflow path is absent.
 19. A duct as claimed in claim 12, wherein theintake port and the output port are provided with necks which extendfrom the shell for connection to a supply and return of heat exchangermedium.
 20. A duct as claimed in claim 12, wherein the heat exchangercomprises a spine formed by inner portions of the first flow path andthe second flow path that extends in a longitudinal direction of theduct.